The fastest way to report a bug is in-game: type /bug in chat to open the report panel (or /bug your description to prefill it). Your world, coordinates, and character are attached automatically, and you can track the report and its resolution under your account's bug reports.
Useful reports include browser or device, screenshots, console errors, and clear reproduction steps.
Preview worlds may reset or change quickly, so reports from test shards should include the build or patch date when possible.